1. Field of the Invention
The present invention relates to a rubbing treatment apparatus, a rubbing method and an alignment film forming method for rubbing a predetermined film-to-be-treated so as to form an alignment film of a device which uses a liquid crystal, such as a liquid crystal display device which is used as display means for OA (office automation) equipment such as a word processor and a personal computer. Further, the invention relates to a rubbing cloth and a rubbing roller of a rubbing treatment apparatus. Still further, the invention relates to a liquid crystal display device including an alignment film which is formed through a rubbing treatment which is performed by the rubbing treatment apparatus.
2. Description of the Related Art
In recent years, as OA equipment such as word processors and personal computers has become popular, an increasing attention is placed on a liquid crystal display device which is characterized In lightness and a low power consumption, for example.
A liquid crystal display device is formed by interposing a liquid crystal between a pair of substrate members, at least one of which transmits light. The pair of substrate members is obtained by forming a display electrode on a light transmitting substrate and thereafter forming an alignment film on the substrate which contains the electrode. In an STN (super-twisted nematic) liquid crystal display device, for instance, liquid crystal molecules which are present between the substrate members are aligned as they are twisted in the range of 180 degrees or larger, e.g., 210 to 260 degrees, between the substrate members. The alignment film has a function of aligning the liquid crystal molecules in one direction. To provide for this function, a surface of the alignment film is rubbed by a predetermined rubbing treatment apparatus.
In recent years, as not only as note type personal computers but as thin display means as well, liquid crystal display devices which provide a dot matrix display of the STN type have expanded a market, due to an improvement of display characteristics. A demand for an increase in the screen size and finer structures has been mounting, and therefore, it is important to improve an in-screen uniformity of a display quality, an in-screen uniformity of a display quality for a half tone display in particular, and a contrast ratio.
Since a display quality is related to alignment of liquid crystal molecules within liquid crystal layers of a liquid crystal display device, a rubbing method and a rubbing treatment apparatus for forming an alignment film, which determines alignment, are extremely important.
FIG. 13 is a plan view for explaining rubbing treatment, showing a substrate member 92 at a step of forming an alignment film. The substrate member 92 which will be eventually divided into four substrate members is prepared, a thin film which is formed at a surface of the substrate member 92 is rubbed.
The substrate member 92 includes a substrate 93, external connection terminals 94, and thin films 96. On one surface of the substrate 93 which has a rectangular shape, four sets of the external connection terminals 94 are formed. Further, adjacent to the external connection terminals 94, four rectangular regions 95 are defined. The four edges of each region 95 are parallel to the respective edges of the substrate 93, and the regions 95 correspond to display areas of the liquid crystal display device. A display electrode is formed inside each region 95. The thin films 96 of an alignment film material such as a polyimide resin are formed in the regions 95 which cover the display electrodes and an exposed surface of the substrate 93.
At a step of rubbing treatment, using a rubbing treatment apparatus which will be described later, rubbing treatment is performed on the thin films 96 of the substrate member 92 which has such a structure, thereby obtaining an alignment film. The substrate member 92 as it is after the alignment film is formed is divided so that each divided portion contains each region 95, whereby the substrate members of the liquid crystal display device are obtained.
The rubbing treatment apparatus for rubbing the thin films 96 of the substrate member 92 comprises a stage for mounting the substrate member 92, and a rubbing roller 97. A rubbing cloth is applied on a cylindrical curved surface of the rubbing roller 97, and the rubbing roller 97 is in a contact with one sur face of the substrate member 92 on t he stage. At this step, rubbing treatment is performed in such a manner that the longitudinal direction of the rubbing roller 97 and a predetermined edge of the substrate member 92 intersect at a relatively large crossing angle.
More specifically, in rubbing, the rubbing roller 97 is rotated in a predetermined rotation direction along a circumferential direction with a central axis of the rubbing roller 97 as a rotation center axis so that the curved surface is rubbed against the substrate member 92 with a predetermined pressure, while moving the rubbing roller 97 and the stage relative to a predetermined direction f or movement 98. At this step, a contacting edge 98a where the curved surface of the rubbing roller 97 contacts with the substrate member 92 is expressed by a straight line whose both ends meet the two perpendicular edges of the substrate member 92, or a straight line whose both ends meet the other two opposite edges of the substrate member 92. The direction for movement 98, which is perpendicular to the direction in which the contacting edge 98a extends, is a direction which is at a predetermined angle .theta.7 with one edge of the substrate member 92. The thin films 96 of the substrate member 92 is entirely rubbed, through the relative motion in the direction for movement 98. A rubbing axis of the alignment film for the substrate members of the liquid crystal display device which is obtained in this method has the predetermined angle .theta.7 with one edge of the alignment film.
In the rubbing treatment described above, the angle .theta.7 is determined with an emphasis on a viewing angle characteristic of the liquid crystal display device. More particularly, with respect to a liquid crystal display device which is used as display means for OA equipment, since a user of the de vice generally sits in front of the liquid crystal display device, an excellent viewing angle characteristic is demanded in a 12-6 o'clock direction of the liquid crystal display device.
Thus, when rubbing treatment is performed wit h the angle .theta.7 of 45 degrees, the length of the contacting edge 98a largely changes during a period from the beginning of the rubbing treatment until the end of the rubbing treatment. This largely changes a contacting surface area of the rubbing roller 97 for the contact with the thin films 96 per unit time, i.e., a substrate area of the substrate member 92 which is processed by rubbing per unit time. A frictional stress which is created by the contact of the substrate member 92 and the rubbing roller 97 is approximately in proportion to the substrate area, and therefore, the frictional stress largely changes during the period from the beginning of the rubbing treatment until the end of the rubbing treatment. More precisely, the frictional stress increases as the substrate area which is rubbed increases, since the beginning of the rubbing treatment until the rubbing roller 97 reaches a central portion of the substrate member 92. Further, after the rubbing roller 97 reaches the central portion of the substrate member 92 until the end of the rubbing treatment, the frictional stress decreases as the substrate area which is rubbed decreases.
When the frictional stress largely changes during the rubbing treatment in this manner, a rotation speed of the rubbing roller 97 which rotates at a predetermined rotation speed changes. That is, the rotation speed of the rubbing roller 97 becomes slow as the frictional stress increases, whereas the rotation speed of the rubbing roller 97 becomes fast as the frictional stress decreases. When the rotation speed changes, the condition of alignment of the alignment film varies, whereby alignment of the liquid crystal molecules becomes irregular and the display quality of the liquid crystal display device accordingly deteriorates.
One control method for preventing a change in the frictional stress during the rubbing treatment is to electrically control the torque, for instance, of a drive system which contains a motor which drives the rubbing roller 97, to thereby keep the rotation speed constant. More precisely, when the rotation speed of the rubbing roller 97 becomes slow as the frictional stress increases, the quantity of electric power which is supplied to the rubbing roller 97 is increased, while when the rotation speed of the rubbing roller 97 becomes fast as the frictional stress decreases, the quantity of the electric power which is supplied to the rubbing roller 97 is decreased, so that the torque of the motor of the drive system is controlled. However, in this method, a delay is easily created between the timing at which the frictional stress upon the rubbing roller 97 changes and the timing at which the torque of the drive system changes so that adjustment of the rotation speed is delayed accordingly, and therefore, it is difficult to sufficiently correct a change in the frictional stress.
Further, since the thin films 96 are formed only in the regions 95 on one surface of the substrate member 92, a boundary between the regions 95 and the remaining region is stepped by an amount equivalent to at least the thickness of the thin films 96. During the rubbing treatment, a portion of the contacting edge 98a passes the remaining region as well. When the contacting edge 98a passes to the remaining region from the regions 95 or vise versa, the pressure between the rubbing roller 97 and the substrate member 92 changes at the stepped portion between the regions 95 and the remaining region, whereby the frictional stress dramatically changes. The change in frictional stress forces rubbing to vary on the thin film 96 in the vicinity of the stepped portion, and therefore, alignment of the liquid crystal molecules in the liquid crystal display device becomes irregular.
As a first conventional technique regarding the liquid crystal display device described above, Japanese Unexamined Patent Publication JP-A 63-124027 (1988) may be cited. In this publication, to prevent a pressure which is applied to a display surface of a liquid crystal element from creating a defect in alignment of liquid crystal molecules in a liquid crystal layer, a rubbing axis of an alignment film is set approximately parallel to the longitudinal direction of a rectangular substrate. In addition, as a second conventional technique, Japanese Unexamined Patent Publication JP-A 5-203955 (1993) maybe cited. In this publication, to prevent a change in a pressure within cells, and hence, cell thicknesses due to movement of liquid crystal molecules in a liquid crystal layer, a rubbing direction for rubbing an alignment film is set approximately parallel to the longitudinal direction of a strip-shaped electrode of a device.
In these conventional techniques, although a rubbing axis and a rubbing direction of an alignment film of substrate members of a finished liquid crystal display device are set, it is not clear how the positions of substrate members and a rubbing roller are arranged during a production stage. Therefore, when the four edges of the region 95 and the four edges of the substrate member 92 are not set parallel to each other during a production stage, the length of the contacting edge 98a largely changes as described earlier. Thus, these two conventional techniques do not consider a frictional stress associated with rubbing, and have different objectives from the invention.
Thus, when surfaces of the thin films 96 of the substrate member 92 are flat, the problem described above arises. In addition, as a demand for an increase in the screen size and finer structures of a finished liquid crystal display device has mounted recently, in order to reduce a load upon a drive voltage because of the resistance of a display electrode, the display electrode tends to be formed into a large film thickness. As a result, a step is created between a surface of a substrate and a surface of the display electrode, and hence, a step in a surface of a thin film which is formed on the display electrode, thereby creating a problem as described below.
FIG. 14 is a plan view for describing a rubbing treatment using a rubbing treatment apparatus according to other conventional technique, showing a substrate member 101 and a rubbing roller 102. In the substrate member 101, display electrodes 104 are formed on one surface of a substrate 103, a film-to-be-treated 105, which is the thin films described above, of a resin such as polyimide is formed covering the display electrodes 104. The rubbing roller 102 is obtained by winding a rubbing cloth 107 around a surface of a roller 106 using a double-coated adhesive tape or the like.
More precisely, while kept in a contact under pressure with a surface of the film-to-be-treated 105, the rubbing roller 102 is rotated about a rotation axis 108 in a predetermined direction 109 at a predetermined speed while concurrently moving the substrate member 101 and the rubbing roller 102 relative to each other, e.g., by moving the rubbing roller 102 in a direction 110, so that the surface of the film-to-be-treated 105 is rubbed in a predetermined direction, and consequently, an alignment film is obtained.
Now, the following conditions are chosen for a liquid crystal display device which provides a dot matrix display. The substrate 103 of the substrate member 101 is rectangular. The electrodes 104 are shaped in the form of a strip, and the longitudinal direction of the electrodes 104 is parallel to one edge 103a of the substrate member 103. The rubbing cloth 107 of the rubbing roller 102 is a cloth to which a pile yarn as described later is implanted, and one of implanting directions of the pile yarns is parallel to the rotation axis 108. Further, during the rubbing treatment, the edge 103a of the substrate member 103 at which the rubbing treatment is started is at a predetermined angle a with respect to the rotation axis 108. A direction which is perpendicular to the one implanting direction of the pile yarns of the rubbing roller 102 is a rubbing direction 111, which is a direction which is perpendicular to the rotation axis 108 here.
FIG. 15A is a plan view of a fabric 112 of the rubbing cloth 107, and FIG. 15B is a perspective view expanding the fabric 112. The fabric 112 of the rubbing cloth 107 is a cloth in which pile yarns 114, which are bundles of several tens of filaments, are woven with foundation yarns 113 which are woven to extend laterally and vertically in such a manner that the implanting lengths of the pile yarns 114 are uniform. Considering an effective use, the fabric 112 is cut into a rectangle along an implanting direction 115a of the pile yarns 114 and a direction 115b which is perpendicular to the implanting direction 115a in such a manner that the length of one edge of the rectangle is the same as the circumferential length of the roller 106 and the length of other edge is the same as the width-direction length of the roller 106, and thereafter, the fabric 112 is wound and adhered around the roller 106 so that the implanting direction 115a is parallel to the rotation axis 108.
FIGS. 16A to 16D are plan views for describing a liquid crystal display device 116 of the STN type which comprises an alignment film which is formed using the rubbing treatment apparatus according to the conventional technique. As shown in FIG. 16A, with respect to the directions of viewing angles with respect to a display screen of the liquid crystal display device 116, an upward direction, a downward direction, a right-hand side direction and a left-hand side direction in the drawing are 12-o'clock, 6-o'clock, 9-o'clock and 3-o'clock directions. A case in which the direction with the largest contrast ratio is set as the 12-o'clock direction, for example, is called 12-o'clock viewing angle setting. The rubbing direction is selected so that the viewing angles distribute the contrast ratio or the like symmetrical between the upward direction and the downward direction and between the right-hand side direction and the left-hand side direction.
In one substrate member 117, as shown in FIG. 16B, for instance, a strip-shaped electrode 119 is formed on a surface of a substrate 118, an alignment film 120 is formed covering the strip-shaped electrode 119, and the strip-shaped electrode 119 is formed in the direction of an edge 118a of the substrate 118 at which the rubbing treatment is started, namely, in a direction which is perpendicular to the edge 118a which is in the 12-o'clock direction. The rubbing treatment is performed with a rubbing direction 111a at an angle .beta.1 with the edge 118a, whereby the alignment film 120 is formed.
In a similar manner, in other substrate member 121, as shown in FIG. 16C, for instance, a strip-shaped electrode 123 is formed on a surface of a substrate 122, an alignment film 124 is formed covering the strip-shaped electrode 123, and the strip-shaped electrode 123 is formed in parallel to an edge 122a of the substrate 122 at which rubbing treatment is started, namely, in parallel to the edge 122a which is in the 12-o'clock direction. Rubbing treatment is performed with a rubbing direction 111b at an angle .beta.2 with the edge 122a, whereby the alignment film 124 is formed.
With respect to the substrate members 117 and 121 which respectively include the alignment films 120 and 124, as shown in FIG. 16D, the alignment films 120 and 124 are faced with each other, the rubbing directions 111a and 111b are at an angle .gamma. with each other, and the substrate members 117 and 121 are adhered to each other with a liquid crystal disposed between the substrate members 117 and 121, whereby the liquid crystal display device 116 is formed. The angles .beta.1 and .beta.2 are both selected as 30 degrees, for example, whereas the angle .gamma. is selected as 240 degrees. The angle .gamma. is a twist angle between the substrate members of liquid crystal molecules which are interposed between the substrate members 117 and 121.
FIG. 17 is an enlarged cross sectional view of a state during rubbing treatment. While the film-to-be-treated 105 is formed to cover the electrodes 104, since the film thickness of the electrode 104 is selected as 2000 .ANG. to 4500 .ANG., for instance and the film thickness of the film-to-be-treated 105 is s elected as about 500 .ANG., for instance, a step due to the electrode 104 is created in the surface of the substrate member 101. The rubbing roller 102 rubs the surface of the substrate member 101 which has such a step, with the pile yarns 114.
FIGS. 18A and 18B are enlarged views of a state during rubbing treatment in the conventional techniques. FIG. 18A is a plan view, and FIG. 18B is a perspective view. The substrate 103 of the substrate member 101 is rectangular, the longitudinal direction of the electrode 104 which is shaped in the form of a strip is parallel to one edge 103a of the substrate 103, one of implanting directions of the pile yarns of the rubbing roller 102 is parallel to the rotation axis 108, and the edge 103a of the substrate 103 at which the rubbing treatment is started is at the predetermined angle .alpha. with respect to the rotation axis 108. Since the step described above is created in the surface of the substrate member 101, during the rubbing treatment, as shown in FIG. 18B, pile yarns 114a, 114b and 114c serially contact one edge 125a of a convex portion which forms the step. Further, the pile yarns 114a, 114b and 114c serially leave an edge 125b which is opposite to the edge 125a. This disturbs bristle edges of the pile yarns 114, and hence, a rubbing direction between a central portion of the electrode 104, a peripheral portion of the electrode 104 at which the rubbing treatment is started, and a peripheral portion of the electrode 104 at which the rubbing treatment ends. As described earlier, since the step tends to be large, the disturbance to the rubbing direction becomes remarkable.
In the liquid crystal display device which comprises the alignment film to which such rubbing treatment is performed, alignment of the liquid crystal molecules becomes irregular within a pixel region in which the electrodes of one substrate member and the other substrate member are mutually superimposed, and therefore, the twist angles .gamma. become irregular. Further, this particularly causes a phenomena known as drop-off that pre-tilt angles of pixels in a peripheral portion of the 12-o'clock direction and a peripheral portion of the 6-o'clock direction become larger than a pre-tilt angle of pixels in the central portion and a transmission rate accordingly increases. The pre-tilt angles are angles of the direction of major axes of the liquid crystal molecules with respect to the surfaces of the substrate members.
Further, originally, larger a difference between a pre-tilt angle with a display voltage applied (i.e., with an ON-voltage applied) to the liquid crystal layer and a pre-tilt angle without the display voltage applied (i.e., with OFF-voltage applied), larger a ratio of transmission rates of light, namely, a contrast ratio becomes large. However, as described above, since the pre-tilt angles are irregular within the screen, the difference between the pre-tilt angle wit h the ON-voltage applied and the pre-tilt angle with the OFF-voltage applied is small, and therefore, the contrast ratio decreases.
In addition, a reduction in OFF-voltage accordingly makes the ON-voltage insufficient, and hence, the transmission rate decreases. When the luminance of back light incorporated in the liquid crystal display device is increased to obtain a sufficient luminance with a high transmission rate, a consumption power increases and a continuous drive time becomes short.
Moreover, as a third conventional technique, a method of reducing a disturbance in a rubbing direction is disclosed in Japanese Unexamined Utility Model Publication JP-U 2-033030 (1990). The publication is related to a technique for winding and adhering a rubbing cloth around a roller, and discloses to arrange a bonding edge without a pile yarn at an angle with respect to a rotation axis of a rubbing roller so that portions which do not contact the pile yarn and are not rubbed are scattered to thereby eliminate a portion with successive irregular alignments.
Further, as a fourth conventional technique, the inventor of the invention has proposed other method of reducing a disturbance to a rubbing direction using a technique for winding and adhering a rubbing cloth around a roller, the same as the publication mentioned above in Japanese Unexamined Patent Publication JP-A 9-73087 (1997). More precisely, the publication discloses to wind a rubbing cloth around a roller in such a manner that one of implanting directions of pile yarns is at an angle with respect to a rotation axis of a rubbing roller to thereby increase the density of the pile yarns which contact a substrate member and enhance the uniformity of a rubbing direction.
Still further, as a fifth conventional technique, Japanese Unexamined Patent Publication JP-A 7-281188 (1995) discloses a technique in which a liquid crystal is aligned stably and uniformly with excellent pre-tilt angles by specifying the roughness of a surface of a rubbing cloth of a rubbing roller. As a sixth conventional technique, Japanese Unexamined Patent Publication JP-A 7-168186 (1995) discloses a technique in which the angles of pile yarns with respect to a fabric surface of a rubbing cloth are specified and a direction of rubbing a substrate member and the angles of the pile yarns are made to coincide with each other so that the uniformity of an alignment of liquid crystal is improved. As a seventh conventional technique, Japanese Unexamined Patent Publication JP-A 2-22624 (1990) discloses a technique of obtaining an excellent alignment of liquid crystal with a high in-screen uniformity by setting an angle of a rotation axis of a rubbing roller with respect to a direction which is perpendicular to a constant direction, in which a substrate member is moved while in a contact with the rubbing roller, in the range of .+-.1 degrees to .+-.45 degrees.
However, any one of the publications cannot solve the inconveniences which are created due to the positional relationship between the substrate member 101 and the rubbing roller 102 described earlier with reference to FIGS. 18A and 18B.